How Water Captured by Leaves Mitigates Storm-Water Runoff

Inside a Franklin & Marshall biology laboratory, junior Karisa (Yiting) Liu and first-year Ann Medina work with a “fog box,” a perspiring, mist-filled plexiglass container in which a small sycamore tree is separated from its planting pot by a sealed plexiglass sheet.

“Interception is the process of trees capturing water from rainfall, and capturing water helps slow down the runoff that causes flooding,” said Liu, a biology and classical languages major. “My project is measuring the interception capacity of the tree—on the leaf canopies and the bark.”

F&M Associate Professor of Biology Sybil Gotsch, who studies the effect of climate change on plants and the ecosystem services that plants provide to communities, directed a visitor’s attention to the fog box.

“The pot and soil are excluded from the misting, so only the branches and leaves are getting wet,” Gotsch said. “Water that adheres to leaf and bark surfaces is called ‘interception,’ the temporary storage of water on plants.”

For their research, Liu and Medina grow small trees in the Steinman Plant Growth Facility on campus and collect samples of bark and twigs from mature and young trees in Lancaster.
Image Credit: Deb Grove

The research is an ongoing project, the professor said. “We’ve studied how water can move through trees and how water is stored in the trunks and roots,” she said. “Now we’re looking at a different variable and that is interception capacity.”

With the fog box, Liu and Medina, her volunteer assistant, measure and record the amount of water the leaves capture. For their research, the students grow small trees in the Steinman Plant Growth Facility on campus and collect samples of bark and twigs from mature and young trees in Lancaster.

“We’re examining some common species of trees in Lancaster to see which ones are better and faster in their process of capturing water,” Liu said.

Inside the fog box, three small nebulizers, electronic minicompressors, sit in a pan of water and make mist. Liu adjusts them to turn the interior into a cloud that produces condensation, which the leaves capture through interception. The water increases the plant’s weight.

“She’s watching the increase in weight of the plant over time,” the professor said. “This little sapling has already reached its maximum and it’s holding about 30 grams of water.”

The plant is small and so the weight is being held by only a small number of leaves. Gotsch said that since sycamores can grow to be large trees, the water that their leaves can collectively intercept is likely to be in the hundreds of liters.

Eventually, Gotsch said, “We will be able to scale these values to relevant sizes of street trees and compare this sycamore to the seven other species that are the most common street trees used in Lancaster.”

Since some trees will have greater interception capacity than others, she said, the research can apply to urban landscape design with a balanced mix of trees for effective storm-water runoff.